Emulsions comprised of a dispersing medium and silver halide microcrystals or grains have found extensive use in photography. Radiation sensitive silver halide emulsions have been employed for latent image formation. The radiation sensitive silver halide grains employed in photographic emulsions are typically comprised of silver chloride, silver bromide, or silver in combination with both chloride and bromide ions, each often incorporating minor amounts of iodide. Radiation sensitive silver iodide emulsions, though infrequently employed in photography, are known in the art. Silver halide emulsions are known to be useful in photographic elements for purposes other than latent image formation, such as for radiation absorption or scattering, interimage effects, and development effects.
In general silver halides exhibit limited absorption within the visible spectrum. Progressively greater blue light absorptions are observed in silver chloride, silver bromide, and silver iodide. However, even silver iodide emulsions appear pale yellow, with their principal light absorption occurring near 400 nm.
The crystal structure of silver iodide has been studied by crystallographers, particularly by those interested in photography. The most commonly encountered crystalline class of silver iodide is the hexagonal wurtzite class, hereinafter designated .beta. phase silver iodide. Silver iodide of the face centered cubic crystalline class, hereinafter designated .gamma. phase silver iodide, is also stable at room temperature. The .beta. phase of silver iodide is the more stable of the two phases so that emulsions containing .gamma. phase silver iodide grains also contain at least a minor proportion of .beta. phase silver iodide grains.
Byerley and Hirsch, "Dispersions of Metastable High Temperature Cubic Silver Iodide", Journal of Photographic Science, Vol. 18, 1970, pp. 53-59, have reported emulsions containing a third crystalline class of silver iodide, the body centered cubic class, hereinafter designated .alpha. phase silver iodide. .alpha. phase silver iodide is bright yellow, indicating that it exhibits increased absorption in the blue portion of the spectrum as compared to .beta. and .gamma. phase silver iodide, which are cream colored. The emulsions containing .alpha. phase silver iodide studied by Byerley and Hirsch were unstable in that they entirely reverted to cream colored silver iodide at temperatures below 27.degree. C.
Daubendiek U.S. Ser. No. 784,139, filed Oct. 4, 1985, commonly assigned, titled ELEMENTS CONTAINING BRIGHT YELLOW SILVER IODIDE discloses silver iodide emulsions exhibiting at temperatures below 25.degree. C. an absorption transition wavelength that is bathochromically displaced by at least 20 nm as compared to the absorption transition wavelength of .beta. phase silver iodide. The emulsions are disclosed to be useful for absorbing blue light.
In silver halide photography one or more silver halide emulsion layers are usually coated on a single side of a support. An important exception is in medical radiography. To minimize patient X ray exposure silver halide emulsion layers are commonly dual coated (that is, coated on both opposed major faces) of a film support. Since silver halide emulsion layers are relatively inefficient absorbers of X radiation, the radiographic element is positioned between intensifying screens that absorb X radiation and emit light. Crossover exposure, which results in a reduction in image sharpness, occurs when light emitted by one screen passes through the adjacent emulsion layer and the support to imagewise expose the emulsion layer on the opposite side of the support. Loss of image sharpness results from light spreading in passing through the support.
It is quite common in radiography to use blue emitting intensifying screens. At the same time radiographic supports used with these screens are typically clear or blue tinted; hence, in each instance transparent to blue light.
A variety of approaches have been suggested to the art to reduce crossover, as illustrated by Research Disclosure, Vol. 184, August 1979, Item 18431, Section V. Research Disclosure is published by Kenneth Mason Publications, Etd., Emsworth, Hampshire P010 7DD, England. More particularly it has been taught to coat a relatively lower speed silver halide emulsion between the support and a higher speed silver halide emulsion layer to reduce crossover, as illustrated by Van Stappen U.S. Pat. No. 3,923,515.
While the art cited above is considered most pertinent to the invention claimed, additional art which may be of background interest is identified and discussed in the Related Art Appendix following the Examples.